1. Technical Field of the Invention
The present invention relates to an improved cylinder head for an overhead-cam internal combustion engine, where the cylinder head has an improved oil passage structure formed therein. The invention also relates to an engine including the improved cylinder head along with a valve actuating mechanism, maintained in an oil bath in a cam chamber defined between the cylinder head and a cylinder head cover. The invention also relates to a vehicle incorporating such an internal combustion engine.
2. Background Art
A number of known designs exist for overhead-cam type internal combustion engines, in which a camshaft assembly, disposed in a cam chamber defined between a cylinder head and a cylinder head cover, is operative to selectively operate intake and exhaust valves, respectively. For example, there has been known an overhead-cam internal combustion engine wherein some of the head bolts, used for coupling the cylinder head and the cylinder block, are disposed outside of the cam chamber (refer, for example, to Japanese Patent No. 3547382, FIGS. 2 and 4).
In the overhead-cam internal combustion engine of this reference, the cam chamber and a space outside the cam chamber (for example, the crank chamber) communicate with each other through a timing chamber in which part of the valve train assembly is disposed. In addition, the valve train assembly includes such a torque transmission part as a timing chain for transmitting the power of the crankshaft to the camshaft in a camshaft assembly of the overhead camshaft type, and a push rod in a camshaft assembly of the push rod type.
Therefore, in the cam chamber, the lubricating oil, having lubricated the camshaft assembly and other members, flows through the timing chamber, to be discharged from the cam chamber outside of the cam chamber. In the cam chamber, however, in areas other than the vicinity of the area where the timing chamber is opened, the lubricating oil is not easily discharged from the cam chamber. In four-wheeled vehicles and similar vehicles, arranged such that the vehicle body is less frequently inclined at a lateral angle as compared with two-wheeled vehicles such as motorcycles, lubricating oil, which has been collected in the cam chamber, is not easily discharged through the timing chamber.
As a result, it is sometimes difficult for the thus-collected lubricating oil to be circulated smoothly in a lubricating oil circulation system composed of an oil pump, an oil passage structure including plural oil galleries, and the like. Therefore, the collected lubricating oil is heated by the combustion heat, which accelerates deterioration of the lubricating oil. In addition, due to the stagnation of the lubricating oil, the effective amount of heat exchange with the cylinder head is reduced, which leads to a lowering in the cooling effect of the lubricating oil.
Further, in the case where a cylinder head, in which the cam chamber is reduced in size by arranging head bolt outside the cam chamber, is to be provided with an oil discharge passage other than the timing chamber, the oil discharge passage must be arranged at such a position as to avoid interference with the intake valve, the exhaust valve and the like members provided in the cylinder head or with the space formed in the cylinder head (for example, the intake port, the exhaust port and, further, the water jacket). Due to the need to secure a space for realizing this arrangement, the cam chamber and, hence, the cylinder cover and the cylinder head cover must remain relatively large in size, which adversely affects the intended size reductions of these components.